[Feature] Custom error messages

From updated **README.md**:
## Custom error messages

In rails you probably want to use `rescue_from Pundit::NotAuthorizedError` in your
controllers and display some nice error page or an informative API response. Pundit
allows you to customize the error messages if authorization fails.

``` ruby
authorize @post, :destroy?, "sorry, we support freedom of speech"
```

You can also provide a custom error message from within a policy, just declare a
method called `error_message` that accepts a `query` parameter and you're good to go.

``` ruby
class MessagePolicy < ApplicationPolicy
  def error_message(query)
    @error_message ||= "whoa there, I don't think you're allowed to #{query} this #{record}"
  end
end